An example of such an ion current detection apparatus disclosed in Japanese Patent Application Laid-Open No. 4-191465 will be described.
As shown in FIG. 6 of the accompanying drawings, an ignition apparatus 2 to which is applied an ion current detection apparatus 100 includes a spark plug 10 provided for each cylinder (only one cylinder is represented in FIG. 6) of an internal combustion engine, as well as an ignition coil 12 for applying the spark plug 10 with high voltage for ignition purpose.
A battery voltage Vb is applied to one end of a primary winding L1 of the ignition coil 12, while the other end of the primary winding L1 is grounded via a power transistor 14, which is turned on and off in accordance with an ignition signal IG. One end of a secondary winding L2 of the ignition coil 12 is connected to a center electrode of the spark plug 10, and the other end of the secondary winding L2 is connected to the ion current detection apparatus 100. An outer electrode of the spark plug 10 is grounded.
In the ignition apparatus 2, when the ignition signal IG is at a high level, the power transistor 14 is turned on, so that a current flows through the primary winding L1 of the ignition coil 12. When the ignition signal IG subsequently reaches a low level and the power transistor 14 is turned off, a high ignition voltage is generated across the secondary winding L2 of the ignition coil 12. This high voltage is applied to the center electrode of the spark plug 10 in order to cause the spark plug 10 to effect spark discharge. The ignition apparatus 2 is designed such that the center electrode of the spark plug 10 attains negative polarity during the spark discharge; therefore, the spark discharge current Isp caused by the spark discharge flows from the spark plug 10 to the secondary winding L2.
The ion current detection apparatus 100 includes a resistor 20, one end of which is grounded; a diode 22 which is connected in parallel to the resistor 20 and whose cathode is grounded; a capacitor 24 connected in series to the ungrounded end of the resistor 20 and to the ungrounded end of the diode 22; and a Zener diode 26 which is connected in parallel to the circuit comprising the resistor 20, the diode 22, and the capacitor 24. The cathode of the Zener diode 26 is connected to the capacitor 24, and the anode of the Zener diode 26 is grounded. The connection line between the capacitor 24 and the Zener diode 26 is connected to the secondary winding L2 of the ignition coil 12. A voltage generated across the resistor 20 is output as a detection value Vio.
In the ion current detection apparatus 100 having the above-described structure, the spark discharge current Isp stemming from spark discharge of the spark plug 10 flows through a current path including the capacitor 24 and the diode 22, while causing the Zener diode 26 to produce a Zener voltage Vz. Therefore, due to the spark discharge current Isp, the capacitor 24 is charged by a voltage Vc (=Vz-Vf) which is smaller than the Zener voltage Vz of the Zener diode 26 by the forward voltage Vf of the diode 22.
When the high ignition voltage induced in the secondary winding L2 drops to a level lower than the Zener voltage Vz, the capacitor 24 starts discharging, so that a high detection voltage according to the charged voltage Vc is applied to the spark plug 10 via the secondary winding L2 of the ignition coil 12. As a result, an ion current Iio flows in accordance with the number of ions generated between the electrodes of the spark plug 10. Since the ion current Iio flows through the resistor 20, the ion current detection apparatus 100 outputs a detection value Vio corresponding to the ion current Iio.
However, in the secondary-side circuit of the ignition apparatus 2, since the inductance of the secondary winding L2 of the ignition coil 12 and the capacitance between the electrodes of the spark plug 10 form a resonant circuit, voltage damped oscillation is generated after completion of spark discharge of the spark plug.
Depending on the operation conditions of the internal combustion engine, the magnitude of the current that flows during that period may reach a value of several to several tens of times the ion current Iio. In addition, the oscillation continues for a relatively long period of time as long as several milliseconds. Therefore, as shown in FIG. 7, the oscillation component is superposed on the ion current Iio, resulting in it being impossible to measure properly the ion current Iio.
In order to overcome the above-described problem, the measurement may be performed at a point in time when the voltage damped oscillation has converged. However, since the charge accumulated in the capacitor 24 is consumed by the voltage damped oscillation, when the voltage damped oscillation converges, a high voltage required for detection of the ion current Iio becomes impossible to obtain, resulting in possible failure to detect the ion current Iio.
This problem can be mitigated through an increase in the capacitance of the capacitor 24, which allows a larger amount of charge to be accumulated during spark discharge of the spark plug 10. However, in this case, if only a small amount of charge is consumed due to flow of the ion current Iio, an undesirable voltage is applied to the spark plug 10 due to the charge remaining in the capacitor 24. In this case, if particles of deposited carbon and liquid fuel are present on the surface of the insulator of the spark plug 10, particles are easily moved and aligned between the electrodes by an electric field that is produced through the voltage application. As a result, there arises a new problem that so-called contamination of the spark plug 10, in which the insulating resistance between the electrodes of the spark plug decreases, occurs quickly.